1. Field of the Invention
The invention relates to high data rate methods and systems for recording and playback of digital video signals. More particularly, the invention relates to a method and system for recording and playing back digital video signals on a video recorder such that an acceptable image is provided during any one of a plurality of trick mode speeds.
2. Description of the Related Art
Significant attention has been paid in recent years to the use of digital compression in video for transmitting and storing digital data. Such digital compression schemes are being studied in applications ranging from low bit rate and low resolution video conferencing, to high bit rate and high resolution high definition television (HDTV). These digital video systems are projected to replace the older well-known analog video systems.
A known limitation of analog TV systems is the practical limitation on the data rate, i.e., the effective rate at which the TV signal carries data or picture and sound information. Analog TV systems produce the TV image by rapidly projecting a series of still images or image frames, in essentially the same way a movie film is projected with a series of frames to create a moving picture. The analog TV signal is divided into segments or frames corresponding to the projected frames of the TV image. The full frame of the analog signal is necessary to construct the corresponding full TV image frame. It is difficult or impossible to transmit the analog frames in a more compact form, for example, by eliminating redundant information from frame to frame.
Digital TV systems overcome these limitations by using a digital signal that includes numerical data for each picture element or pixel of the TV image frame. The digital signal is segmented into a bit stream, the bit stream including a series of numbers, ones and zeros, corresponding to the pieces of information organized into data "frames." The organization of these pieces of data within the frame is referred to as the format of the frame. Parenthetically, a digital frame does not correspond one-for-one to a full frame of the TV image. Usually, one digital frame corresponds to only a relatively small portion of a TV image frame. The word "frame" as used in the remainder of this document refers to a digital frame unless otherwise indicated.
Digital technology offers a number of advantages, probably the most important of which is error detection and correction features. The information in the digital signal also can be processed to reduce or eliminate redundancies from frame to frame. Moreover, the digital information can be coded (using a single symbol or small group of symbols to represent a larger number or set of numbers) to increase the efficiency of the data transfer and correspondingly increase the data transfer rate. A processor in the digital TV set can be used to decode the data and use it to project the desired TV image. Techniques for increasing the efficiency of the data while decreasing the redundancy are known as data compression techniques.
Various data compression techniques for coded data have been proposed. A well-known coded data structure is the Moving Picture Expert Group (MPEG) encoding strategy, which is described in "Information Technology--Coding of Moving Pictures and Associated Audio (MPEG-2)," ISO-IEC 13818, International Standards Organization, Geneva, 1993, the contents of which are incorporated herein by reference. The MPEG encoding format reduces redundancies by including various types of data in each data frame, including intraframe (I) data, predictive frame (P) data, and bidirectionally-coded frame (B) data. I-data is stand-alone data that is unique to a frame. P-data and B-data is data that relates to or links two or more frames, e.g. looking back to a previous frame for the desired data, such as motion vector data or residual data. Moreover, the I-data, P-data, and B-data are positioned at random locations within each data frame.
The structure of the compressed digital data, however, creates some new problems which did not exist in the older analog technology. For example, applying digital technology to VCR equipment, and particularly to VCR equipment that is practical for consumer markets, presents a number of challenges. Most importantly, the cost of the systems must be relatively low for market acceptability. The systems also must be able to perform "trick" modes (modes other than normal playback and record), such as various speeds of fast forward (FF) or fast reverse (FR), to enable a customer to browse a tape. It is further desirable that VCRs have various speeds of FF and FR.
The helical scan mechanical recording systems of commercially available VCRs usually are the most expensive component of the VCR system. Commercially available helical scan systems used in most consumer VCRs today have a bandwidth of about 8-15 MHz, and can record or play back at rates up to about 35-40 Mb/s. These helical scan systems are designed to operate with analog TV signals, but they are capable of operating with digital signals as well, at least for data rates that are relatively low. Helical scan systems and rotary heads for VCR applications that are capable of operating at significantly higher speeds commensurate with the full digital signal transmission rates are available, but they are prohibitively expensive for consumer systems. Therefore, it is desirable to have a digital VCR system that uses commercially available, low cost helical scan systems or their substantial equivalents, but which operates at the highest practical data rate.
As used in VCRs of the current analog design, these commercially available, low cost helical scan systems perform trick modes by correspondingly changing the rotational speed of the heads and the translational speed of the recording tape. It is well known that, as the speeds change from the normal recording or playback speeds, the orientation of the moving heads relative to the recorded tracks on the tape cause the heads to scan past the tracks with a steep diagonal trajectory, rather than the normal playback trajectory. For example, FIGS. 1A and 1B show a tape 40 with compressed digital data stored sequentially in consecutive tracks. Solid arrow 42 indicates the trace of the VCR recording/scanning head in a normal play trajectory. The resulting output of the scan will be the same as the input of the scan, assuming the absence of tape errors. Arrow 44 indicates the head scan during one FF mode, while arrow 46 indicates the head scan during one FR mode. These trajectories disturb the signal and the resulting image because the resulting signal comprises only bits and pieces of adjacent frames, rather than the full frames.
In the old analog system, because the analog frames recorded on the tracks of the tape are in analog form, these bits and pieces when combined are sufficient to produce an image that is acceptable to the viewer, although somewhat disrupted. This accounts for the horizontal lines that appear in the video image during fast forward, for example. However, when compressed digital signal formats with variable length coded data are used, standard rotary heads cannot read the data directly during trick modes. This is because the data within the frames is not predominantly the same from frame to frame as in an analog signal. As the heads scan the bits and pieces of the digital tracks, they obtain an unusable mixture of I-data, P-data, and B-data from various frames. The bits and pieces, when used as a signal and displayed, do not produce a viewable TV image. As different speeds of FF and FR are selected (not shown in FIG. 1) no image results because the resulting bitstream is not decodable.
In an effort to solve this problem, proposals have been made to position data frames along the tracks of a recording medium, particularly positioning the I-data within the frames, such that the recording/scanning head, when travelling along a trick mode trajectory, will scan I-data. These related proposals suffer from at least one of the two following drawbacks.
First, most VCRs operate with a primary FF and a primary FR speed; however, many VCRs can also operate with several other FF and FR speeds besides the primary speed. Most related attempts to position I-data to be read in trick mode trajectories typically work only in the primary FF and FR speeds. Others provide an image at several FF and FR speeds, but it is greatly degraded in comparison to the image provided at the primary FF and FR speeds. None of the prior attempts enable sufficient I-data to be read at all FF or FR speeds to consistently reproduce an acceptable image.
Second, the prior attempts required the VCR record/scanning head to return to a preselected position, e.g. to the beginning of a tape track or to one of the select set of tracks, in order to see the I-data in the FF or FR trajectories. The prior attempts did not enable the record/scanning head to commence its FF or FR trajectory from any position, and still see sufficient I-data to produce a viewable picture.